Use IDs instead of paths in Markdown upload URLs

What does this MR do and why?

Changes URLs to markdown uploads to use project / group IDs instead of project / group paths

This uses the route introduced in !151744 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

How to set up and validate locally

1. Enable use_ids_for_markdown_upload_urls feature flag
2. On any Markdown field, upload a file
3. Click on the "Preview" tab to see the generated HTML
4. The URL for the upload should use the new format with IDs instead of paths